Field of the Invention
The present invention relates to an image pickup element and an endoscope device.
Description of the Related Art
In recent years, in fields of medical applications such as endoscopes, fluorescence agents which can be administered to the human body for observing lesions such as cancer or blood flow have been applied. The most common fluorescence agent which can be administered to the human body is a fluorescence agent called indocyanine green (ICG), which excites and emits fluorescence in an infrared wavelength region.
A technique capable of capturing a normal visible image and an infrared image with a fluorescence wavelength of ICG at the same time is known (for example, refer to Japanese Patent Publication No. 3962122). FIG. 14 is a block diagram which shows a configuration of a conventionally known fluorescence endoscope device capable of capturing a normal visible image and an infrared image with a fluorescence wavelength of ICG at the same time. In the example shown in FIG. 14, a fluorescence endoscope device 1A causes visible light to be incident on RGB image pickup elements 26, 27, and 28 using a dichroic mirror 22. As a result, the RGB image pickup elements 26, 27, and 28 generate image signals. An infrared light component is reflected by the dichroic mirror 22, passes through an excitation light cut filter 23, and is incident on an infrared image pickup element 25. Accordingly, the infrared image pickup element 25 generates an ICG fluorescence image signal.
FIG. 15 is a graph which shows characteristics of a band-pass filter 12 disposed in a light source device 3A of the conventionally known fluorescence endoscope device 1A. A line 1501 is a line which shows transmittance of the band-pass filter 12. With these characteristics, the band-pass filter 12 transmits a visible component of light of a xenon light source 11 and an excitation wavelength component of an ICG fluorescence substance and emits the components toward a subject.
FIG. 16 is a graph which shows characteristics of the dichroic mirror 22 of the conventionally known fluorescence endoscope device 1A. A line 1601 is a line which shows transmittance of the dichroic mirror 22. With these characteristics, the dichroic mirror 22 is configured to transmit a visible component and reflect an infrared component.
FIG. 17 is a graph which shows characteristics of the excitation light cut filter 23 of the conventionally known fluorescence endoscope device 1A. A line 1701 is a line which shows transmittance of the excitation light cut filter 23. With these characteristics, the excitation light cut filter 23 is configured to transmit only components with a wavelength longer than a fluorescence wavelength of the ICG fluorescence substance so that the infrared image pickup element 25 can image only a fluorescence image.
FIG. 18 is a graph which shows characteristics of the excitation wavelength and the fluorescence wavelength of the conventionally known ICG fluorescence substance. A line 1801 is a line which shows intensity of excitation light. A line 1802 is a line which shows intensity of fluorescence. In order to generate an ICG fluorescence image signal, it is necessary to remove the excitation wavelength component emitted toward a subject by a light source and to image light with only a fluorescence wavelength component using an image pickup element.
With the above configuration, the fluorescence endoscope device 1A can image a visible image and a fluorescence image at the same time by separating visible light and infrared light using the dichroic mirror 22 and causing them to pass through the excitation cut filter 23. The fluorescence endoscope device 1A is a system which can perform image pickup by emitting light with a visible component from the light source device 3A and light with an excitation light component from a laser light source 7 to a subject.
In addition, a technology related to a hybrid PD-PD imager is known (for example, refer to U.S. Patent Application, Publication No. 2013/0075607). FIG. 19 is a cross-sectional view which shows a conventionally known image pickup element of a hybrid structure in which two layers of the image pickup element are stacked and image pickup is performed in the lower layer using light that has passed through the upper layer. In an example shown in FIG. 19, a first substrate 221 and a second substrate 222 are stacked. First photodiodes 223-1 to 223-n are formed on the first substrate 221 of the upper layer. Second photodiodes 224-1 to 224-n are formed on the second substrate 222 of the lower layer.
With this configuration, light transmitted through the first photodiodes 223-1 to 223-n formed on the first substrate 221 of the upper layer can be received by the second photodiodes 224-1 to 224-n formed on the second substrate 222 of the lower layer. Accordingly, it is possible to perform image capturing at the first photodiodes 223-1 to 223-n formed on the first substrate 221 and the second photodiodes 224-1 to 224-n formed on the second substrate 222 at the same time.
Light incident on silicon, which is a material widely used as a CCD or CMOS image pickup element, is absorbed at a deeper point when it has a longer wavelength. Accordingly, if the first substrate is set as a thin image sensor like a BSI using a hybrid PD-PD imager as shown in FIG. 19, a component with a long wavelength like the ICG fluorescence wavelength described in a graph of FIG. 18 can pass through the first substrate 221 and can be imaged in the second substrate 222. In this case, compared to the configuration described in FIG. 14, a dichroic mirror is not necessary and a camera head can be decreased in size.